Flyback capacitor level shifter feedback regulation for negative pumps

ABSTRACT

Systems and methods of flyback capacitor level shifter feedback regulation for negative pumps. In accordance with a first embodiment of the present invention, a feedback regulator for a negative output charge pump comprises a flyback capacitor for inverting an output of the negative output charge pump to a positive voltage. The feedback regulator further comprises a voltage comparator for comparing the positive voltage to a reference voltage. The voltage comparator is also for producing an enable signal for control of pump driving signals to the negative output charge pump. The feedback regulator further comprises a first plurality of switches for selectively coupling a first terminal of the flyback capacitor between a low voltage and the output and a second plurality of switches for selectively coupling a second terminal of the flyback capacitor between a low voltage and the voltage comparator. Further, the feedback regulator comprises switch control logic for controlling the plurality of switches.

RELATED APPLICATION

This Application claims benefit of U.S. Provisional Patent ApplicationSer. No. 60/556,002, filed Mar. 23, 2004, entitled “Flyback CapacitorLevel Shifter Feedback Regulation Scheme for Negative Pumps” to VijayKumar Srinivasa Raghavan, which is hereby incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

Embodiments in accordance with the present invention relate generally toelectronic circuits, and more particularly to fast and low power flybackcapacitor level shifter based feedback regulation circuits, as may beused in charge pump systems.

BACKGROUND

Charge pump circuits are widely used by electronic designers. Chargepumps may be configured as both positive and negative systems. Given apositive reference voltage (for example, a bandgap reference voltage),negative charge pump systems are generally required to level shift thenegative output voltage to a positive feedback voltage for comparisonwith the positive reference voltage for closed loop feedback regulation.

A negative pump system typically comprises a series of charge pumpcells. The output of the pump is level shifted and compared with areference voltage to generate an enable signal for the pump clock drivesystem. The pump clock drive signal drives the charge pump cells.

In the conventional art using a positive reference voltage, the outputof the pump is shifted to a positive voltage so that it can be comparedwith the (positive) reference voltage in order to control regulation.Conventional art circuits use, for example, resistor divider stackswherein the highest potential voltage is the positive reference voltageand the negative potential is used as the negative feedback voltage. Asupply voltage generally cannot be used as the highest potential in sucha resistor divider stack as the negative pump output voltage would thenvary with variations in supply voltage. Such variations are especiallyunsuitable for applications operating under wide supply voltagevariations.

Hence, a positive reference voltage is used as the highest potential inthe resistor divider stacks of conventional art negative pump systems.Using this technique, a designer can tap a positive level shiftedversion of the negative feedback voltage that can then be compared witha positive reference in the closed loop system. For such a system tohave a desirably fast response to supply voltage variations, theresistor stack should have a low resistance. For example, the referencevoltage serving as the highest positive potential in the resistordivider stack must be able to supply current. Unfortunately, typicalvoltage references are neither designed nor capable of supplying suchcurrent loads. For example, such typical voltage references aregenerally high impedance sources.

To overcome such shortcomings, conventional art systems typically use anoperational amplifier buffer in the reference voltage path in order toprovide such current loads. Utilizing an operational amplifier in such amanner generally requires an operational amplifier with very fastresponse characteristics in order for the charge pump feedback system tohave desirably fast response and turn-on characteristics.

SUMMARY OF THE INVENTION

Systems and methods of flyback capacitor based level shifter feedbackregulation for negative pumps, characterized as having a fast responseand operating with low current, for converting a negative output voltageto a level shifted positive value are highly desired.

Accordingly, systems and methods of flyback capacitor level shifterfeedback regulation for negative pumps are disclosed. In accordance witha first embodiment of the present invention, a feedback regulator for anegative output charge pump comprises a flyback capacitor for invertingan output of the negative output charge pump to a positive voltage. Thefeedback regulator further comprises a voltage comparator for comparingthe positive voltage to a reference voltage. The voltage comparator isalso for producing an enable signal for control of pump driving signalsto the negative output charge pump. The feedback regulator furthercomprises a first plurality of switches for selectively coupling a firstterminal of the flyback capacitor between a low voltage and the outputand a second plurality of switches for selectively coupling a secondterminal of the flyback capacitor between a low voltage and the voltagecomparator. Further, the feedback regulator comprises switch controllogic for controlling the plurality of switches.

In accordance with another embodiment of the present invention, apositive voltage corresponding to an output of the negative outputcharge pump is generated. The positive voltage is compared to a positivevoltage reference from a high impedance source. Pumping of the negativeoutput charge pump is disabled if the positive voltage is greater thanor equal to the positive voltage reference.

Advantages of embodiments in accordance with the present inventioninclude providing an innovative technique to obtain fast and accurateconversion of a negative voltage to its absolute value (correspondingpositive value) so that it can be used in a negative pump feedbacksystem with positive reference voltages. Advantageously, this novelmethod does not require current sourcing capability of the referencevoltage, thus making the fast/non-low power operational amplifier basedsystem unnecessary. An additional advantage is that the feedback levelshifter circuit is no longer a major factor in the pump turn-on timesince no operation amplifiers are employed.

Embodiments in accordance with the present invention are characterizedas having a fast response and operating with low current, overcomingnumerous disadvantages of the conventional art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic of a novel flyback capacitor levelshifter feedback regulation circuit for negative pumps, in accordancewith embodiments of the present invention.

FIG. 2 illustrates an exemplary method for operating a negative outputcharge pump, in accordance with embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of the present invention, flybackcapacitor level shifter feedback regulation for negative pumps, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. However, it will be recognizedby one skilled in the art that the present invention may be practicedwithout these specific details or with equivalents thereof. In otherinstances, well-known methods, procedures, components, and circuits havenot been described in detail so as not to unnecessarily obscure aspectsof the present invention.

Flyback Capacitor Level Shifter Feedback Regulation for Negative Pumps

Embodiments in accordance with the present invention are described inthe context of design and operation of integrated semiconductors. It isappreciated, however, that elements of the present invention may beutilized in other areas of electronic design and operation.

FIG. 1 illustrates a schematic of a novel flyback capacitor levelshifter feedback regulation circuit 100 for negative pumps, inaccordance with embodiments of the present invention. So as to betterillustrate the function of feedback regulation circuit 100, FIG. 1additionally illustrates a charge pump clocking circuit 98 of well knowndesign and a negative output charge pump circuit 99 of well knowndesign. The pump clock drive signals clk1 and clk2, e.g., differentphases of a same frequency, of charge pump clocking circuit 98 drive thenegative output charge pump circuit 99.

The output voltage of negative output charge pump circuit 99,illustrated as V-101, is coupled to feedback regulation circuit 100.Feedback regulation circuit 100 is provided a reference voltage, Vref102. Reference voltage Vref 102 is used as a comparison voltage tocontrol the output V-101 of negative output charge pump circuit 99.Reference voltage Vref 102 is typically a positive voltage, andgenerally of lower absolute value than the absolute value of outputV-101. Advantageously, for example, reference voltage Vref 102 may be abandgap reference voltage of about 1.25 volts.

Feedback regulation circuit 100 comprises four switches, 110, 120, 130and 140. Feedback regulation circuit 100 further comprises a flybackcapacitor 150 and a voltage comparator 160. Feedback regulation circuit100 optionally comprises a capacitor or other scaling circuitry 170.Switch logic 180 provides control signals to control the action of fourswitches 110, 120, 130 and 140. Switch logic 180 is typically driven atthe pump clocking rate, e.g., via pump clock signal 102. Pump clocksignal 102 typically oscillates at the same rate as the pump clockingsignals. It is to be appreciated, however, that a duty cycle and/orphase relationship may differ between pump clock signal 102 and the pumpclocking signals, e.g., clk1 or clk2.

The function of feedback regulation circuit 100 is now described. Thenegative pump output V-101 is level shifted using a flyback capacitortechnique. During a first portion of the pump cycle, switches 110 and120 are controlled on, e.g., closed or conducting, while switches 130and 140 are off, e.g., open or non-conducting. This arrangement ofswitches causes the “bottom” plate of the flyback capacitor 150, e.g.,the terminal coupled to switch 110, to charge to the V-101 voltage.

In a second portion of the pump cycle, switches 110 and 120 arecontrolled off, while switches 130 and 140 are on. This causes thebottom plate of the flyback capacitor 150 to be coupled to Vss and thetop plate of the flyback capacitor 150 to go to a positive voltage equalto the absolute (positive) value of the negative output voltage. Theabsolute value of the negative voltage can further be scaled viacapacitor or other scaling circuitry 170 to compare with the referencevoltage Vref 102 supplied to the system.

When switch 140 is closed, the voltage on flyback capacitor 150, a“flyback voltage,” is coupled to an input of voltage comparator 160. Itis appreciated that optional scaling circuitry 170 may reduce thevoltage originally present on flyback capacitor 150. A second input ofvoltage comparator 160 is coupled to the reference voltage, Vref 102. Itis to be appreciated that no buffering stage, e.g., an operationalamplifier, need be placed between the source of the reference voltage102 and the comparator 160. As discussed previously, such bufferingstages are deleterious to startup and feedback performance of voltageregulating circuitry.

When the flyback voltage is less than the reference voltage, the outputof voltage comparator 160, enable signal 161, enables charge pumpclocking circuitry 98 to pump charge pump 99. This action generallycauses charge pump 99 to increase the magnitude of its output voltage.

When the flyback voltage is greater than the reference voltage, theoutput of voltage comparator 160, enable signal 161, disables chargepump clocking circuitry 98. Hence, charge pump 99 is not pumped, and themagnitude of its output voltage is not increased.

The signal(s) generated by switch logic 180 that control the action ofthe switches should be non-overlapping in such a way that there is noleakage of charge from the flyback capacitor 150. The switches shouldrun at the input pump clock frequency and hence the value of thenegative output voltage is updated once every clock cycle. Since thiscircuit does not require sinking current from the reference voltage,there is advantageously no requirement for a current source capabilityof the input voltage reference. Further, a high current operationalamplifier buffer with fast response characteristics is advantageouslynot needed, as the input is of high impedance.

A disadvantage of conventional art negative pump systems that utilizehigh current operational amplifier buffers is that such high currentoperational amplifier buffers can take approximately five microsecondsjust for startup. This means that during pump start-up, for example, theoutput of the high current operational amplifier buffer will not bevalid for the first five microseconds. Such a system will have totalnegative pump startup time of around ten microseconds including startuptime of the high current operational amplifier buffer and pumping time(based on a 20 MHz pump). Also, the current consumption of such highcurrent operational amplifier buffers and current sinking resistordivider stack based level shifter is around 300 micro amps.

In contrast, the charge pump circuit 100 of an embodiment in accordancewith the present invention may be used in applications where the turn-ontime requirement is a maximum of six microseconds. The flyback capacitortechnique of level shifting does not involve an operational amplifier,and as a result it can level shift the negative output voltage in lessthan 25 nanoseconds, even after accounting for non-idealities inswitching transistors. The total startup time for such a negative pumpsystem, including the total of flyback level shifter regulation startuptime and pumping time (20 MHz pump clock), is on the order of fivemicroseconds. Further, the current consumption of the flyback levelshifter regulation circuit is around 50 micro amps.

FIG. 2 illustrates an exemplary method 200 for operating a negativeoutput charge pump, in accordance with embodiments of the presentinvention. In 210, a positive voltage is generated corresponding to anoutput of the negative output charge pump. The generating may comprisecharging a first terminal of a capacitor in a first portion of a cyclewith the output of the negative output charge pump and coupling a secondterminal of the capacitor to a low voltage and coupling the firstterminal of the capacitor in a second portion of the cycle, to the lowvoltage and accessing the positive voltage at the second terminal of thecapacitor. The low voltage may be a local ground reference, e.g.,chassis ground, for the charge pump.

In 220, the positive voltage is compared to a positive voltage referencefrom a high impedance source. The high impedance source may comprise abandgap reference. In accordance with an alternative embodiment of thepresent invention, the positive voltage reference is not buffered.

In 230, pumping of the negative output charge pump is disabled if thepositive voltage is greater than the positive voltage reference.

In this novel manner, the negative output charge pump is regulated bycomparing its output voltage to a reference voltage.

Advantages of embodiments in accordance with the present inventioninclude providing an innovative technique to obtain fast and accurateconversion of a negative voltage to its absolute value (correspondingpositive value) so that it can be used in a negative pump feedbacksystem with positive reference voltages. Advantageously, this novelmethod does not require current sourcing capability of the referencevoltage, thus making the fast/non-low power operational amplifier basedsystem unnecessary. An additional advantage is that the feedback levelshifter circuit is no longer a major factor in the pump turn-on timesince no operation amplifiers are employed.

Embodiments in accordance with the present invention are characterizedas having a fast response and operating with low current, overcomingnumerous disadvantages of the conventional art.

Embodiments in accordance with the present invention, flyback capacitorlevel shifter feedback regulation for negative pumps, are thusdescribed. While the present invention has been described in particularembodiments, it should be appreciated that the present invention shouldnot be construed as limited by such embodiments, but rather construedaccording to the below claims.

1. A feedback regulator for a negative output charge pump comprising: aflyback capacitor for inverting an output of said negative output chargepump to a positive voltage; a voltage comparator for comparing saidpositive voltage to a reference voltage and for producing an enablesignal for control of pump driving signals to said negative outputcharge pump; a first plurality of switches for selectively coupling afirst terminal of said flyback capacitor between a first low voltage andsaid output; a second plurality of switches for selectively coupling asecond terminal of said flyback capacitor between a second low voltageand said voltage comparator; and switch control logic for controllingsaid plurality of switches.
 2. The feedback regulator of claim 1 whereinsaid enable signal controls said pump driving signals.
 3. The feedbackregulator of claim 1 wherein said switch control logic operates at thesame frequency as said pump driving signals.
 4. The feedback regulatorof claim 1 wherein no more than one switch of said first secondplurality of switches is in a conductive condition at a time.
 5. Thefeedback regulator of claim 1 further comprising a scaling circuit toscale said positive voltage prior to input to said voltage comparator.6. The feedback regulator of claim 5 wherein said scaling circuitcomprises a capacitor.
 7. The feedback regulator of claim 1 wherein saidlow voltage comprises a local ground reference for said feedbackregulator.
 8. A method of operating a negative output charge pumpcomprising: generating a positive voltage corresponding to an output ofsaid negative output charge pump; comparing said positive voltage to apositive voltage reference from a high impedance source: disablingpumping of said negative output charge pump if said positive voltage isgreater than said positive voltage reference wherein said generatingcomprises: in a first portion of a cycle, charging a first terminal of acapacitor with said output of said negative output charge pump andcoupling a second terminal of said capacitor to a low voltage; and in asecond portion of said cycle, coupling said first terminal of saidcapacitor to said low voltage and accessing said positive voltage atsaid second terminal of said capacitor.
 9. The method of claim 8 whereinsaid cycle corresponds to said pumping of said negative output chargepump.
 10. The method of claim 8 wherein said first and second portionsof said cycle do not temporarily overlap.
 11. The method of claim 8wherein said low voltage comprises a local ground reference for saidnegative output charge pump.